The present invention relates to an automatic lathe. More particularly this invention concerns an apparatus for feeding profiled bars to a screw machine.
A screw machine is an automatic lathe through whose headstock workpieces to be machined can be fed in. Typically such machines are used for the serial production of turned workpieces.
In order to maximize the efficiency with which such a machine operates it is standard practice to provide it with an automatic feed device. Such an apparatus has a magazine of bars which are to be turned down in the screw machine and with means for advancing them sequentially through the chuck of the machine into a desired position. The feed apparatus as described in commonly assigned copending patent application Ser. No. 594,285 (now U.S. Pat. No. 4,006,654) whose entire disclosure is also herewith incorporated by reference, has a bar-holding clamping sleeve which grips the rear end of the bar and is connected via a bearing to a pusher. A chain drive or the like advances this pusher along the rotation axis of the headstock so that a bar engaged in its clamping sleeve can be advanced axially through the headstock, from the back thereof. Once the bar has passed through the headstock the collet thereof is clamped on it and the machining operation can take place.
Such an arrangement is relatively simple so long as the bars are of round cross-sectional shape. It has, however, been found very useful in practice to employ bars of non-round cross-sectional shape. Frequently square or otherwise polygonal sectional are employed in order to facilitate subsequent machining, as the facets of such workpiece need not subsequently be milled into it. In such arrangements the collet invariably has a hole of the same cross-sectional shape as the bar by which is meant the collet closely engages one or more facets of the bar and has an aperture which is not round. Thus a triangular aperture can be used with triangular, hexagonal, or nonagonal shapes. Complementary collet apertures and bar sections ensures excellent driving of the bar by the collet so that machining is greatly facilitated.
The disadvantage of using such nonround-section bars is that the bar must be exactly aligned with the collet aperture in order to fit the bar through this aperture. In a manual loading operation this is simply achieved by manually pressing the front end of the bar against the back end of the arrested collet and turning the bar until it fits through the collet. The relatively light pressure needed for such manual interfitting of the two parts eliminates the possiblity of damaging either the collet or the bar during such loading.
The various known feed devices for polygonal-section bars almost all require a separate arrangement for arresting the collet in a predetermined angular position. Not only does this increase the cost of the screw machine, but it slows down the loading operation considerably as the often massive collet must be brought to a complete stop in a predetermined position before the bar advance operates. Furthermore, such an arrangement is not readily applicable to existing screw machines which are not provided with a collet-stopping arrangement.
It has also been suggested to provide a feed apparatus which automatically rotates the bar before insertion into the collet at the same speed as the collet and only presses to the two together when the speeds are substantially the same. Such an arrangement is extremely complicated and expensive. Furthermore such an arrangement cannot readily be adapted for use with existing screw machines.
Various prior-art feed devices are shown in German Pat. No. 2,055,904, in German utility models Nos. 1,993,771 and 6,911,266 (U.S. Pat. No. 3,602,075)